The head is sharply marked off from the rest of the body, and the mouth
is surrounded by a circle of arms bearing suckers or claw-like hooks.
These arms are supposed to be a modification of part of the foot, the
rest of this organ being constituted by a hollow, funnel-shaped
structure, which places the mantle-cavity in communication with the
exterior. The water used for respiration is forced suddenly out from the
mantle-cavity, through the funnel, so as to propel the animal in the
opposite direction. A gland, known as the “ink sac,” secretes a brownish
fluid, which may be pressed out of the sac so as to shield the animal
from attack. All cephalopods are marine, and, since they are therefore
of no agricultural importance, it is unnecessary to enter into details
regarding their form and structure, and I will only mention the
following: =Pearly Nautilus= (_Nautilus pompilius_), with external shell
and numerous arms; =Paper Nautilus= (_Argonauta argo_), with a very
thin, boat-shaped shell, and eight arms; =Poulpe= (_Octopus_), without
shell, and with eight arms; =Cuttle-fish= (_Sepia_), and =Squid=
(_Loligo_), also without external shell, and possessing ten arms.